Slurry dewatering and conversion of biosolids to a renewable fuel

ABSTRACT

In the processes for treating municipal sewage and storm water containing biosolids to discharge standards, biosolids, even after dewatering, contain typically about 80% water bound in the dead cells of the biosolids, which gives biosolids a negative heating value. It can be incinerated only at the expense of purchased fuel. Biosolids are heated to a temperature at which their cell structure is destroyed and, preferably, at which carbon dioxide is split off to lower the oxygen content of the biosolids. The resulting char is not hydrophilic, and it can be efficiently dewatered and/or dried and is a viable renewable fuel. This renewable fuel can be supplemented by also charging conventional biomass (yard and crop waste, etc.) in the same or in parallel facilities. Similarly, non-renewable hydrophilic fuels can be so processed in conjunction with the processing of biosolids to further augment the energy supply.

CROSS REFERENCE TO RELATED APPLICATION APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/025,544, now U.S. Pat. No. 8,409,303, filed Feb. 11, 2011, which is acontinuation of U.S. patent application Ser. No. 11/269,499, now U.S.Pat. No. 7,909,895, filed Nov. 7, 2005, which claims priority to U.S.Provisional Patent Application Ser. No. 60/626,680, filed Nov. 10, 2004,the disclosures of which are hereby expressly incorporated by referenceherein in their entirety.

FIELD OF THE INVENTION

Sludge from sewage and wastewater treatment plants, and the biosolids itcontains, represents a serious disposal problem. The Water EnvironmentFederation (WEF) formally recognized the term “biosolids” in 1991, andit is now in common use throughout the world. The WEF defines“biosolids” as the soil-like residue of materials removed from sewageduring the wastewater treatment process. During treatment, bacteria andother tiny organisms break sewage down into simpler and more stableforms of organic matter. The organic matter, combined with bacterialcell masses, settles out to form biosolids. According to USEPA,biosolids that meet treatment and pollutant content criteria “can besafely recycled and applied as fertilizer to sustainably improve andmaintain productive soils and stimulate plant growth”.

The sludge is a mixture of biosolids (comprised primarily of deadorganic cells which are a by-product of treating sewage and wastewaterso that it can be released into open waters) and varying amounts of freewater. Free water can be at least partially removed by mechanicaldewatering methods. In addition to the free water, the biosolids containcell-bound water, which can make up as much as 80% of the volume ofbiosolids and is impossible to remove by mechanical dewatering methods.The large amounts of water contained in such sludge give it a highlynegative heat value which makes the cost of incinerating it prohibitivebecause large amounts of costly fuel would be required to drive off thecell-bound water. In view thereof, such sludge is presently used aslandfill or as a fertilizer that can be spread over land, because sewagesludge frequently contains nitrogen and phosphor, for example. However,the sludge also contains harmful substances, generates undesirableodors, and can lead to serious contamination of the soil or the landfillfrom, amongst others, heavy metals.

In the processes for treating municipal sewage and storm water todischarge standards, solid constituents are concentrated into aby-product, often referred to as sewage sludge. Sewage sludge is a massor agglomeration of dead organic cells and other solids, calledbiosolids, which are mixed with varying amounts of water ofcorrespondingly varying viscosity. Irrespective of the degree to whichthe mass of biosolids is mechanically dewatered, the remaining mass ofbiosolids typically contains about 80% water, because much of the wateris bound inside the dead cells, giving the biosolids mass a negativeheating value, thereby making biosolids effectively useless for purposesof extracting heat value from them. Thus, biosolids still are disposedof in landfills or by spreading them on agricultural land as afertilizer that supplies nitrogen and phosphorus. However, biosolidsalso may contain live viruses and pathogens and toxic heavy metals,inspiring heated opposition from environmental interests, while theirhigh water content drastically increases the cost of transporting themto a point of use.

According to the present invention, the raw biosolids are heatedfollowing their discharge from the sewage treatment plant to rupture thecells, thereby releasing the large quantities of cell-bound water. Thetemperature is sufficiently high so that the cell structure is destroyedand carbon dioxide is split off to lower the oxygen content of thebiosolids. This results in the formation of char that is not hydrophilicand can be efficiently dewatered and/or dried. This char is a viablerenewable fuel.

In a further development of the present invention, it is possible toincrease the availability of renewable fuels by converting biomass (suchas untreated yard and crop waste, etc.) in the same or parallelfacilities. Similarly, non-renewable hydrophilic fuels can be soprocessed to further augment the energy that can be extracted frombiosolids in accordance with the invention.

BACKGROUND OF THE INVENTION

There is a growing wave of public support for renewable energy popularlycalled “Green Power”. Several well-known companies, according to Powermagazine for May 2003, including General Motors, IBM, Dow Chemical andJohnson & Johnson, have announced plans to purchase a portion of theirpower requirement from “green” sources. Some companies have evenannounced intentions to replace all of the electricity used in theirmanufacturing with “green power”. Pillars of fossil energy supply, suchas Chevron, British Petroleum (BP) and Shell Oil, have announced theirintentions to support environmental causes. In fact, BP is an importantsupplier of solar energy panels. There is a “Green Power MarketDevelopment Group” of the World Resources Institute (WRI), aiming todevelop 1,000 Megawatts (MW) of new, cost-competitive “green power” by2010.

In addition, more than a dozen state legislatures require powermarketers to phase in specific and increasing percentages of power fromrenewable sources. New York has mandated that state agencies must buy25% of their power from renewable sources by 2013; currently 19.3% ofthe energy produced in New York is generated from renewable sources (NewYork Public Service Commission). California has passed legislationrequiring that 20% of utilities' electricity in the state be producedfrom renewable sources by 2017. In fact, one California utility, PacificGas and Electricity (PG&E), advertises that more than 30% of itselectricity now comes from renewable sources. At least 36 U.S. powerretailers now offer a “green power” alternative. Europe also takesrenewable energy seriously, targeting 20% of its generation fromrenewables by 2020.

Conventional renewable energy generally covers origination from solar,wind, hydro-electric, geothermal, biomass and landfill gas. There issome question as to how the demand for renewable energy will be met.Solar and wind are growing, but from a very small base. Hydro-electricand geothermal have limited new sites and face ecological opposition.Landfill gas is limited and also criticized for air pollution. There arecurrently no other renewable sources which might be tapped to fill thelarge gap between supply and demand.

Biomass has long been used as a renewable energy source. For example,wood and forestry, as well as agricultural, by-products have been usedas fuels for centuries by mechanically firing them in furnaces andboilers with high excess air and low efficiency. The National RenewableEnergy Laboratory (NREL) defines biomass as: “organic matter availableon a renewable basis. Biomass includes forest and mill residues,agricultural crops and wastes, wood and wood wastes, animal wastes,livestock operation residues, aquatic plants, fast growing trees andplants and municipal and industrial wastes.” According to The SandiaNational Laboratory's Combustion Research Facility (CRF), combustion isinvolved in 85% of the world's energy use. If biomass is to make ameaningful contribution to renewable energy, it will be, directly orindirectly, as a fuel.

Sewage sludge, and the large amounts of biosolids it contains, withtheir cell-bound water, has not previously been considered an energysource. Due to their large bound water content, biosolids have anegative fuel value and cannot be incinerated unless heated withexpensive fuel that must be purchased. Such an incineration of biosolidsmay be desirable to avoid having to spread them on land, therebyeliminating or at least reducing possible environmental contamination,but at a very substantial cost, namely the additional heat that mustcome from the fuels to incinerate them.

The production of biosolids in the U.S. is estimated to be between 7.1and 7.6 million (short) dry tons per year. Ocean dumping has beenprohibited since the 1980s. The predominant disposition is spreading thebiosolids on agricultural land as a fertilizer. Other dispositions aredumping in landfills and incineration.

In 1998, the production of biosolids in Europe was reported to be 7.2million dry metric tons, and 25% was disposed to landfills. Productionis expected to increase to at least 9.4 million metric tons in 2005,land application accounting for 54%, landfilling decreasing to 19%, andincineration growing to 24%—although incineration is estimated to costfive times as much as landfilling.

In 2001, biosolids production in Japan was reported to be 1.7 milliondry metric tons. 40% was composted and the remainder was incinerated orused to produce cement.

After strenuous mechanical dewatering and digestion in sewage treatmentplants, the solids concentration in biosolids still only ranges fromabout 14-30%, and is typically no more than about 20%, which means thatevery ton of biosolids, treated and dewatered in accordance with theprior art, is accompanied by about four tons of water, the bulk of whichis bound in the dead cells. The cost of shipping the inert water limitsthe distance it can be moved from its source, usually a wastewatertreatment plant (WWTP). These factors give biosolids a negative value.As a result, the WWTP must pay to have someone dispose of the biosolids.Such a payment is often called a “tipping fee”.

As the options for biosolids disposal become more challenging and thedisposal options are moved farther from the source, disposal costs andtransportation costs have become increasingly significant economicburdens. To reduce this burden, industry has focused on volume andweight reduction. The wastewater industry has made extensive efforts toremove the water from the biosolids generated at treatment plants. Atypical WWTP may employ centrifuges, belt presses, rotary presses orother processes to physically force the water from the biosolids. Apolymer and other chemicals may be added to assist in dewatering.Nevertheless, such mechanical dewatering methods used by WWTPs areinefficient and costly and incapable of appreciably reducing the amountof water bound in the cells of the biosolids.

The U.S. Environmental Protection Agency (EPA) grades biosolidsaccording to regulation “40 CFR Part 503” as Class A and Class B. Thisregulation concerns primarily the application of biosolids toagricultural land, to which there is vocal and growing environmentalopposition. For example, environmentalists condemn the use of biosolidsas a fertilizer because of their content of living disease-causingorganisms (pathogens and viruses) and heavy metals (such as lead,mercury, cadmium, zinc and nickel), as well as their damage togroundwater quality. In addition, environmentalists raise concerns about“quality of life” issues, such as insects and odors, associated withbiosolids. As such, land application of Class B biosolids is banned in anumber of counties, and more counties and states are expected to follow.In one case, where 70% of the biosolids were Class B, the banning ofland application in adjacent counties nearly doubled the tipping feefrom about $125 per dry ton to about $210-$235.

Furthermore, the high cell-bound water content of biosolids makes theirincineration difficult for many industries. For example, the cementindustry is reputed to be the world's third largest energy user. Itrequires the equivalent of about 470 pounds of coal to make each ton ofcement. To conserve fossil fuel, 15 cement plants in the U.S. burnfuel-quality hazardous waste, and about 35 other plants use scrap tiresto supplement fossil fuel. A growing method of disposing of biosolids isto incinerate them in cement kilns. Since their net fuel value isnegative, this practice is only viable because of the revenue receivedby the kiln operator, for example, from the tipping fee, sinceadditional fuel, such as coal, must be fired to eliminate the waterbound in biosolids. In addition, in the manufacture of cement, certainelements contained in biosolids, such as chlorine, phosphorus, sodiumand potassium, are not desired because they adversely affect the qualityof the cement.

In the past, the requirement to dispose of biomass in general wascoupled with attempts to extract heat energy from it in order to reducedisposal costs and the environmental burden of landfills. Attempts toextract energy from such materials were limited to combusting low-gradefuels and solid waste. For example, previous processes for deriving fuelfrom municipal solid waste (MSW) generally focus on adding alkali toassist in the removal of the majority of contained chlorine in the formof PVC found in MSW. In addition, various methods for processingrelatively low-grade carbonaceous fuel, such as sub-bituminous andlignite coals, are known to those of ordinary skill in the art. In bothscenarios, however, low-grade fuels are used as raw materials.

A number of schemes for the pyrolysis of biosolids have been advanced.However, they all have been forced to contend with the fact thatbiosolids contain about four times as much water as solid material, evenafter conventional dewatering at the treatment plant, for example. It isimpossible to reach pyrolysis temperatures until all of the water hasbeen vaporized, which requires at least 4000 Btu per pound of solids,which, at best, might be equal to its fuel value, before allowing forcapital and operating costs.

As the foregoing demonstrates, the disposal of biosolids has becomeincreasingly expensive and controversial. A need exists in the art for amethod to cleanly and economically dispose of biosolids. The currentinvention provides a method to dispose of biosolids while concurrentlyproducing an economically more viable renewable fuel.

To the extent that biosolids alone cannot meet the growing demand forrenewable energy, the biosolids conversion to a useable fuel inaccording with the present invention can be combined with extractingenergy from other sources such as biomass. Thus, the present inventionprovides a method and system to convert biosolids, alone or withbiomass, into a viable renewable fuel in an environmentally benignmanner.

SUMMARY OF THE INVENTION

As understood by applicants, biosolids are composed primarily of deadcells which have cell-bound water. When subjected to sufficient pressureto keep the water liquid, a heating of a slurry or sludge containingbiosolids to a first, relatively lower, temperature causes the biosolidscells to rupture, which liberates the water bound inside the cells andthereby converts the biosolids from a substance which cannot practicallybe dewatered to a new fuel from which the water can be readily removedmechanically. The further heating of the biosolids additionally splitsoff carbon dioxide, thereby lowering the oxygen content of the biosolidsand converting the biosolids into char. Once dewatered, the char has apositive heating value and can be used directly as a fuel, therebyreleasing the heat energy that was previously inaccessibly bound in thebiosolids.

For example, in combination with a wastewater treatment plant (WWTP),the present invention provides a method to produce a viable, renewablefuel from biosolids by converting the biosolids into a relatively dry,combustible material. In many cases, the process can be integrated withthe existing infrastructure of the WWTP. Since the treated biosolidshave substantially no bound water, freed water from the cells can bereturned to the WWTP. The remaining cell materials become much lesshydrophilic, which gives them a positive heating value and allows themto be shipped to the desired destination at a much reduced cost. If theWWTP is equipped with an anaerobic digestion stage, the gas produced cansupport the fluid deoxidation with fuel used in its operation. Pathogensare destroyed, and when the dewatered biosolids are heated sufficientlyto carbonize them, the resulting char product contains reduced levels ofmost water-soluble impurities, including sodium, potassium, sulfur,nitrogen, chlorine and organic compounds, which are separated with theexcess water. Biosolids char is a new player on the energy scene and isa low-cost, renewable fuel for many energy-consuming industries.

Although acceptable to incinerators and landfills, biosolids char ismost productively used where its energy content is utilized. Forexample, in one embodiment, the method and system of the presentinvention is used in conjunction with a cement kiln to increase thethermal efficiency of the cement kiln and cement manufacture, whiledisposing of biosolids that would otherwise constitute undesirablewaste. In addition, the inert material found in the biosolids can becomea part of the product. Therefore, not only is the heating value inbiosolids utilized, but the inerts leave no negative by-products fromthis configuration.

Since biosolids are produced as a viscous slurry, little preparation isrequired, except blending for uniformity. Following heating of thebiosolids slurry under pressure to a temperature at which the cell wallsrupture, the further heating of the biosolids results in a significantmolecular rearrangement of the cells, splitting off of a substantialproportion of its oxygen as carbon dioxide, which carbonizes organicsubstances and yields so-called char that is readily incinerated. Thetemperature necessary for this molecular rearrangement varies but istypically between 177° C. and 315° C. (350° F. and 600° F.). Theaggressively hydrolyzing conditions free anions to dissolve in theaqueous phase. Previously bound cations, such as sodium and potassium,are likewise made accessible to aqueous dissolution and subsequentremoval and/or disposal.

Compared to the incineration of (raw) biosolids, in cement kilns ordedicated incinerators, the positive energy content of biosolids charsubstantially decreases the amount of supplemental fuel which must bepurchased. Moreover, soluble cations, sources of low temperature slag inboilers and undesirable in cement, have been largely removed with thefreed water.

Cement kiln and incinerator facilities may prefer for the char to bedewatered to a maximum extent, resulting in the delivery and charging ofa wet solid “char” containing only about 40% to 50% water, which isabout one-fifteenth of that found in the raw biosolids. Alternatively,transport and handling considerations may cause such facilities toprefer char which has been dried and compacted or pelletized. Thepresent invention has the capability to deliver char in either form.

In addition, the biosolids char produced with this invention, with orwithout char from other substances such as biomass, for example,provides a fuel that is useful to a variety of other fuel-consumingindustries, including blast furnaces, foundries, utility boilers, thepower industry, the paper industry, and other fossil fuel-utilizingindustries. For example, the present invention contemplates a greenpower station where biosolids char is charged to a pulverized fuel orfluidized bed combustor to generate steam, or to a gasifier feedingclean fuel gas to an integrated gas-fired gas turbine combined cycle.

Furthermore, the char produced by the present invention can be the rawmaterial for hydrogen fuel cells through partial oxidation to a fuel gas(largely carbon monoxide and hydrogen), followed by the water gas shiftand the separation of carbon dioxide, as practiced in synthetic ammoniatechnology. It can be “refined” into liquid fuels by adaptations of“catalytic cracking”, “delayed coking” and “hydrocracking”, patternedafter the established processes well known to the petroleum refiningindustry.

While the present invention is directed to the economic and ecologicallysound disposition of biosolids, it can be combined with appropriatelytreated other substances, primarily biomass that requires disposal,including, but not limited to, paper mill sludge, food waste,agricultural wastes, hog manure, chicken litter, cow manure, rice hulls,bagasse, green waste, municipal solid waste, medical waste, paper waste,wood and wood waste, palm oil residue, refuse derived fuels, Kraft Millblack liquor, and short rotation energy crops, as well as hydrophilicnon-renewable fuels such as low-rank coals.

In particular, the present invention relates to a process of convertingbiosolids into an economically viable fuel by applying sufficientpressure to the biosolids to maintain liquidity, heating the pressurizedbiosolids to a sufficient temperature to rupture cells and then toevolve carbon dioxide, depressurizing the resulting char slurry,separating the carbon dioxide from the char slurry, and removing atleast a portion of the aqueous phase from the char slurry to provide anat least partially dewatered char product for further use. Additionally,the invention relates to reacting the dewatered char product with a gascomprising oxygen to thereby convert its fuel value into thermal energyand using the thermal energy or incinerating the fuel.

In sum, the present invention provides an environmentally acceptabledisposition of biosolids, as well as energy for various energyconsumers, such as cement kilns and electric power plants. In addition,the present invention provides: (a) a method to increase theavailability and environmental acceptability of renewable fuels; (b) amethod to minimize the quantity of wastes to be landfilled; (c) aprocess to reduce the moisture (water) content of waste going tolandfill; (d) a process to raise the softening point of renewable fuelash to reduce fouling and slagging; (e) a method of converting anon-uniform solid fuel, such as agricultural and forestry waste and/orpaper mill sludge, into a uniform fuel; (f) a method to convert a bulkyfuel into a fuel that is compact and easy to store and transport; (g) aprocess to convert a perishable fuel into a sterile fuel that isstorable without deterioration; (h) a method to provide an economicalmeans of co-firing an otherwise non-compliant fuel; (i) a method toprovide a thermally efficient combination of liquid deoxidation and atleast one of a wastewater treatment plant, a cement kiln, and a thermalpower station; (j) a method to dry biosolids prior to introduction to acement kiln or other similar facility; (k) a method to reduce the amountof water introduced to a cement kiln and other combustors; (l) a processto co-process multiple feedstocks utilizing fluid deoxidation; (m) amethod to utilize the ash in biosolids and other biomass; (n) a methodto remove (and recover) elements found in biosolids or other biomasssuch as phosphorus, chlorine or CO₂; and (o) a process to remove thewater from biosolids and biomass in order to further refine thesematerials or to reduce disposal costs or to utilize for fertilizer.

Thus, the present invention provides a method for the disposal of sludgegenerated at sewage and wastewater treatment plants in an economical andenvironmentally benign manner. The method is economically benign becausethe end product is ash that is free of odors, as well as harmfulsubstances such as viruses or pathogens, and the ash has a small volumeand is readily disposed of The method is further economically viablebecause at the front end it benefits from the willingness of treatmentplant operators to pay a tipping fee in order to dispose of thedifficult-to-handle sewage sludge, and further because, at the other endof the cycle, the sludge will have been converted into a fuel with apositive heating value that can be used to generate further revenue orother items of value in the form of payments for the generated heatenergy or, for example, trading the extracted heat for credits, desiredproducts and the like.

Additional embodiments of the present invention will be apparent fromthe description and the drawings of this application.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention can be ascertained fromthe following detailed description that is provided in connection withthe drawing(s) described below:

FIG. 1 is a schematic flow diagram illustrating the process of thepresent invention for converting biosolids into a high energy densityslurry or dry solid fuel as a renewable energy source;

FIG. 1A is a schematic flow diagram similar to FIG. 1 illustratinganother process of the present invention for converting biosolids into ahigh energy density slurry or dry solid fuel as a renewable energysource;

FIG. 2 is a flow diagram in which the process of the present inventionis used in a wastewater treatment plant;

FIG. 3 is a flow diagram in which the process of the present inventionis used in the operation of a cement kiln;

FIG. 4 is a flow diagram in which the process of the present inventionis employed in the operation of a thermal power station using additionalfuels, such as low-rank coals; and

FIG. 5 is a flow diagram in which the process of the present inventionis combined with a thermal dryer and used in a cement kiln.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates the conversion of biosolids to a viable renewablefuel. Biosolids may be delivered as sludge via a pipeline 107 from anadjacent wastewater or sewage treatment plant (WWTP) to a raw feed tank106. Alternatively, biosolids may be delivered by a truck 108 and pumpedby a sludge pump 109 via a line 110 to the tank 106.

Alternatively, the raw feed tank 106 can receive biosolids from multiplesources and be utilized as a mixing vessel whereby more dilute biosolidsare mixed with thicker, more viscous biosolids to render a more pumpablefeed. A commingling and slurrying facility 104 can also be utilized forthis purpose.

In addition, the raw feed tank 106 or comminuting commingling andslurrying facility 104 can be the points whereby polymer is added toreduce the water content of the biosolids slurry or, alternatively,where water is added if the viscosity of the slurry is an issue.

Heat may be added to the tank 106 to enhance the viscosity of thebiosolids. In addition, a shearing or grinding step may be added, forinstance between the raw feed tank 106 and a pumping device 111. Thisshearing or grinding will lower viscosity as well as achieve theparticle size uniformity necessary for optimal operation of a pressurelet-down valve 116. Addition of heat, shearing and grinding will alsoenhance the performance of the pumping device 111 and allow a highersolids content material into the system.

In one embodiment, a screening device is added to remove largeparticle-size items to enhance the performance of any grinding, thepumping device 111 and/or the pressure let-down valve 116. For example,the screening device may be placed between the raw feed tank 106 and thepumping device 111. In another embodiment, the raw feed tank 106 or asimilar device can be used to add a chelating agent or other suitablechemical to remove phosphorus or other elements found in the biosolids.

From the raw feed tank 106, the biosolids slurry is pumped to a pressurethat will keep the water in the slurry in liquid phase during subsequentheating operations. For example, in one embodiment, the slurry is at apressure ranging from about 400 to 1200 psi. In another embodiment, thepressure of the slurry is between about 250 to 1600 psi. Care must beexercised to provide a pumping device 111 with an adequate net pumpsuction head (NPSH), either hydraulically or by mechanical assistance,as with a screw conveyor, considering that the slurry may be veryviscous and may carry dissolved gases.

An alternative (not shown), for reducing the service of the pumpingdevice 111, is the addition of booster pumps in the process anywherebetween the pumping device 111 and the let-down valve 116. A furtheralternative (not shown) for reducing the service of the pumping device111 is the addition of freed water or reacted slurry before the pumpingdevice 111.

The biosolids slurry is pumped through heat exchangers 112 and 113before passing to a reactor 114. While passing through the heatexchanger 112 the slurry is heated by exchange with hot liquid heattransfer fluid (HTF), such as Therminol 59. In another embodiment (notshown), the slurry may be heated via heat exchange with steam, eitherdirectly or indirectly. The outlet temperature of the slurry leavingheat exchanger 112 may range from about 150° C. to 315° C. (300° F. to600° F.), and is preferably between about 200° C. to 260° C. (400° F. to500° F.). While passing through the heat exchanger 113, the slurry isfurther heated to the desired temperature at which the biosolids cellwalls will rupture and to liberate water bound in the cells. Thetemperature is further preferably set so that the other constituents ofthe biosolids cells are carbonized to convert these constituents to charby heat exchange with hot liquid HTF. In an alternate embodiment, thecondensing vapor of a vaporizable HTF, such as Therminol VP-1, is usedto heat the slurry to the desired temperature. In one embodiment, thistemperature is between about 200° C. to 260° C. (400° F. to 500° F.). Inanother embodiment, the temperature is between about 150° C. to 260° C.(300° F. to 500° F.). In still another embodiment, the temperature isbetween about 260° C. to 350° C. (500° F. to 650° F.).

While the design of the heat exchangers for use with the presentinvention is not critical, each may comprise two or more shells. Theshells may be in parallel or in series. In one embodiment, the heatexchangers 112 and 113 are arranged in a series such that the biosolidsslurry passes through heat exchanger 112 prior to heat exchanger 113.

The reactor 114 (which may comprise one or more reactors in parallel orseries) provides time at elevated temperature to first rupture thebiosolids cells and further to complete the deoxidation reactions toconvert the cell constituents to char. While a continuous reaction isdiscussed here, the present invention also contemplates a batch orsemi-batch reaction. As known to those of ordinary skill in the art, themethods for heating the batch reactors can be similar to those for acontinuous reactor. For example, a batch reactor may be heated by directsteam injection, heating coils, or a combination thereof.

One suitable alternative (not shown) for reactor 114 is areactor-stripper tower. Such a tower has side-to-side baffles (or othervapor-liquid contacting media) arranged for downflow of partially heatedslurry from the exchanger 112 contacting an upflow of steam and strippedcarbon dioxide from a “reboiler” (the equivalent of exchanger 113),receiving char slurry from the base of the tower. The tower preferablyhas a top-to-bottom temperature gradient from approximately the slurryoutlet temperature of the exchanger 112 to a temperature somewhat lowerthan that leaving the illustrated simple reactor. In one embodiment, thetemperature gradient ranges from about 200° C. to 260° C. (400° F. to500° F.). In another embodiment, the temperature gradient is betweenabout 150° C. to 315° C. (300° F. to 600° F.). The carbon dioxideleaving the top of the reactor-stripper contains appreciable water vaporthat needs to be condensed in a new condenser to distilled water andseparated from carbon dioxide which leaves via a line 118. While thelet-down valve 116 and separator 117 are still required, little carbondioxide remains to be separated in the separator.

The slurry leaving the reactor (or reactors), referred to as charslurry, consists of destroyed biosolids cells from which the bound waterhas been freed and which has also undergone fluid deoxidation, i.e. amolecular rearrangement characterized by splitting off carbon dioxide,resulting in a substantial increase in solids carbon content and asubstantial decrease in solids oxygen content. For example, char samplesare comprised of about a 2% to 15% increase in solids carbon content,preferably with about a 4% to 12% increase. In one embodiment, thesolids oxygen content decreases by about 35% to 50%. In anotherembodiment, the slurry undergoes a decrease in solids oxygen content ofabout 30% to 70%.

Char slurry flows from the reactor 114 to the heat exchanger 115, whereit is partially cooled by giving up heat to the liquid HTF which comesto it from the exchanger 112 via a line 142. In one embodiment, the charslurry is cooled to a temperature ranging from about 150° C. to 200° C.(300° F. to 400° F.). In another embodiment, the temperature of the charslurry after leaving heat exchanger 115 is between about 100° C. to 260°C. (200° F. to 500° F.). The liquid HTF circuit is completed by a liquidHTF receiver 139, a liquid HTF pump 140 and connecting lines 141, 142and 143.

The services of the heat exchangers 112 and 115 (FIG. 1) could beperformed by a single exchanger 160 (FIG. 1A) having the cold feedslurry on one side and the hot char slurry on the other, which wouldrequire passing slurries through both the tube and the shell sides. Anydeposits on the tube side of the heat transfer service would berelatively easy to clean. Fouling on the shell side would be difficultto correct, however, and heat transfer coefficients are much lower witha product-to-product exchanger. As such, the present inventioncontemplates dividing the service into two exchangers, with clean HTFbeing a “go-between”, both hot and cold slurries then being on the tubesides, with only clean HTF on the shell sides. The duties of the twoexchangers are essentially the same (differing only by radiation loss),the temperature ranges of the circulating HTF seeking their ownequilibrium.

In one embodiment, reacted biosolids char leaving the reactor 114, whilestill under pressure, is recycled via recycle line 162 back to thepressurized biosolids slurry before it enters the reactor 114, as shownin FIG. 1A, in order to facilitate heating and reduce the viscosity ofthe slurry prior to biosolids cell destruction and subsequentdeoxidation.

Vaporized HTF flows from a receiver 144, through a line 145, to the hotside of the exchanger 113, in which it is condensed by the transfer ofheat to partially heat the biosolids slurry, and then flows by means ofa line 146 back to the receiver 144. Liquid HTF flows from the receiver144 by natural convection (or a furnace charge pump, not shown, ifpressure drop requires this) through the coils of a fired heater 147,where it is partially vaporized by heat supplied by a fuel source 148and flows back to the receiver 144. In one embodiment, the fuel sourceis natural gas, propane, fuel oil, char slurry, char, or any combinationthereof. In an alternate embodiment (not shown), a combustion device,such as a fluid bed, is employed to use char, char slurry, or acombination of char and an outside fuel source or waste source. Inanother embodiment (not shown), a gasifier is employed to use char, charslurry, or a combination of char and an outside fuel source or wastesource. In yet another embodiment, a boiler is used to generate steamfor process heat. The boiler could use char, char slurry, or acombination of char and an outside fuel source or waste source.

An HTF pump 149 takes suction from the bottom of the receiver 144 andcirculates liquid vaporizable HTF to a facility 135 as a source of heatfor char drying. After serving this purpose, it is returned, via line150, to the receiver 144. The pump 149 may also serve other auxiliaryheating services (not shown) such as to a jacket for the reactor 114 toprevent heat loss.

After being partially cooled in the heat exchanger 115, the now-fluidchar slurry flows through a cooler 119, in which its temperature islowered to near ambient by exchange with plant cooling water from a line120. The cooled char slurry flows from cooler 119 to an automaticpressure let-down valve 116, which has been responsible for maintainingthe aqueous slurries under sufficient pressure to avoid vaporization.The pressure let-down valve 116 reduces the pressure of the char slurryto a nominal pressure above atmospheric. This is achieved by liberatinggaseous and dissolved carbon dioxide, which is separated from the charslurry in a separator drum 117. Evolved carbon dioxide exits theseparator drum 117 via aline 118.

The pressure let-down valve 116 is subjected to strenuous conditions andhas a high potential for clogging. Certain steps can be performed,however, to minimize these difficult conditions. For example, aspreviously mentioned, grinding or screening can be performed anytimebefore the pressure let-down valve 116. In addition, a step prior to thepressure let-down valve 116 of further cooling the reacted slurry afterthe heat exchanger 115, as shown, will reduce the amount of evolved gasand reduce the acceleration of particles across the pressure let-downvalve 116. Those of ordinary skill in the art will appreciate thatseveral cooling techniques are suitable for use with the presentinvention. Cooling techniques could include counter-current shell andtube or double-pipe exchanger cooled by plant cooling water.

Because foaming may occur in either the storage tank 121 or the drum117, it may be advantageous to control foaming by letting down pressurein two or more stages. In another embodiment, foaming may be controlledby using a spray nozzle from the lower part of the drum 117 to spray aside stream into the drum 117.

Some dissolved carbon dioxide separates in the tank 121 and leaves via aline 137. If there is a use or market for carbon dioxide, this gas,along with that evolved in the drum 117, leaving via the line 118, maybe subjected to purification. Otherwise, it will be collected anddischarged through the flame of a fired heater 147 to destroy traces ofodor-causing gases and/or for energy recovery. Approximately 25 to 27pounds of carbon dioxide are released per ton of wet biosolidsprocessed. Any sulfur compounds in the carbon dioxide will be treatedwith the necessary pollution control devices. All vent gases areconducted to the fired heater 147 to destroy traces of odor-causinggases.

Liquid char slurry flows from the bottom of the tank 121 to a dewateringfacility 122, where one or more commercially-available devices for themechanical separation of liquids and solids is employed to separate thefreed water from the char solids. Suitable separation devices mayinclude, but are not limited to, thickeners, hydroclones, centrifuges,pressure and vacuum rotary filters, horizontal filters, belt and rotarypresses, and the like.

Liquid char slurry in the tank 121 will contain some heat and may beideal for a further step of adding a chelating agent or other chemicalsto remove phosphorus or other elements found in the original biosolids.The chelating agents discussed above are also suitable for use at thisstage in the process.

Char solids leave the dewatering facility 122 via a conveyance means123. Some or all of them may be directed to an eductor 124 in which theyare mixed with sufficient water from a line 125 to form a pumpable, highenergy density fuel slurry. The fuel slurry is accumulated in a tank 126for off-loading to a pipeline or tank truck, as required, by means of afuel slurry pump 151 and a line 152. Alternatively, the damp char may beconveyed by conveyance means 127 and 128 to a damp char hopper 136 to beoff-loaded, as required, into hopper-bottom trucks 156.

Alternatively, part or all of the char leaving the dewatering facility122 can be directed to a drying and/or pelletizing facility 135 viaconveyance means 127, which, utilizing commercially-available equipment,dries and compacts or pelletizes the solids. Heat required for thedrying is supplied by a stream of hot liquid HTF from a vaporizable HTFreceiver 144 by a HTF pump 149 which, after providing the necessaryheat, is returned, via line 150, to the receiver. Dried char fuel isaccumulated in a dried char silo 153, to be off-loaded to hopper-bottomtrucks 155 and transported to market. In one embodiment (not shown),dried char fuel is cooled prior to being accumulated in the dried charsilo 153. In another embodiment, the dried product is stored undernitrogen blanket to prevent dust explosions and fire in the event thatthe product is not transported directly from the facility. Evaporatedwater from the dryer 135 flows through a condenser 138, and thecondensate is transported via a line to the freed water tank.

In one embodiment (not shown), the heat required for the drying facility135 can be produced by at least one of the methods of a fluid bed,boiler, or combusting gas from a gasifier. The fuel source for the heatrequired for the drying could be at least one of char, char slurry, or acombination of char and an outside fuel source or waste source. In oneembodiment (not shown), the gas from a digester at an adjacentwastewater treatment plant is utilized as fuel for at least one of theprocess heater and the dryer.

Although not shown in FIG. 1, nor entirely renewable, char dried in thedrying facility 135, but not pelletized, may be diverted to a mixingdevice with which it is incorporated into a fuel oil. The technologyresembles that of the coal-oil mixture (COM) programs developed andtested in the 1980s. While not conforming to existing fuel oilspecifications, such an addition would add heating value and, in somecases, reduce the sulfur content at low cost. This new fuel is ofinterest for users where ash is not a problem, such as in cement kilnsand blast furnaces. Although any grade of distillate or residual fueloil can be used, most likely candidates are off-spec slop oils, refineryfuel, used lube oil, and the like. The oil-char slurry is alsoattractive for in-plant fuel uses.

Freed water separated from damp char in the facility 122 flows through aline 129 to a freed water tank 130, from which it is pumped by a freedwater pump 131, either via a line 132 to a comminuting and slurryingfacility 104 and/or tank 106, and/or it is returned to the wastewatertreatment plant (WWTP) via line a 134. Depending on the rate scale fortreatment at the WWTP, it may be economical to employ some pretreatment,by known commercial means, in a pretreatment facility 133. Any sludgewhich is derived from the pretreatment facility can be conveyed to thedrying facility 135. As discussed earlier, the dried product may bestored under nitrogen blanket or other method to prevent dustexplosions.

While the process flow diagram of FIG. 1 has been described with respectto the treatment of large amounts of biosolids, as accumulate mostfrequently at municipal sewage and wastewater treatment plants, those ofordinary skill in the art will appreciate that other substances, such asbiomass, can be dewatered with the general process of the invention inaddition to the biosolids to enhance the amount of fuel being generated.For example, fluid biomass wastes, such as papermill and paper recyclingsludges, may be charged via a tank truck 108 or a pipeline 107 or a pump109 and line 110. If the waste contains appreciable amounts of chlorinecompounds, alkali of at least the chemical equivalent of the chlorine isalso added (not shown). Solid biomass wastes, as from agriculture andforestry, may be charged via a conveyor 101 to the comminuting andslurrying facility 104, employing known technology described, forexample, in U.S. Pat. No. 5,685,153, the entire disclosure of which isincorporated by reference herein. Low-grade carbonaceous fuels, such asPowder River Basin sub-bituminous coal, may alternatively oradditionally be charged to the facility 104 via a conveyance means 102.Recycled water is added to the facility as required for specified slurryviscosity by means of the line 132, and/or fresh water by means of theline 103. As outlined above with respect to the biosolids, the slurriedhydrophilic feedstock is transferred via a line 105 to the storage tank106.

The high reactivity of the biosolids char, as produced by a unitexemplified by FIG. 1, has been noted. This property of its carbonaceousmolecules will be useful to a gasification facility, or a chemical plantusing it as raw material for oxygenated organic compounds, either lowmolecular weight (such as acetic acid, alcohols, aldehydes and ketones)or higher molecular weight detergents, surfactants, plasticizers,lubricating oil additives, and the like. Among the future possibilitiesfor char gasification is the shifting of the CO content of the gas tocarbon dioxide and hydrogen, with subsequent separation of the carbondioxide to yield hydrogen for fuel cells. This separation may well beperformed by the new metal-ceramic membranes being developed for theU.S. Department of Energy (DOE) FutureGen project, in collaboration withOak Ridge National Laboratory and Eltron Research.

FIG. 2 is a flow diagram of a combination of a wastewater treatmentplant (WWTP) operating in accordance with the present invention and,adjacent thereto, an efficient biosolids processing facility operatingin accordance with the present invention and employing fluid deoxidationto economically convert biosolids into a combustible material, resultingin the elimination of most of the water from WWTP biosolids, andparticularly the water bound in the biosolids cells, that otherwiseinflate the cost of transporting and/or evaporating the water from thebiosolids and thereby make the use of biosolids unfeasible. Combustiblegas from the WWTP's anaerobic digestion may be used to provide heatneeded for the deoxidation, thus saving the cost of purchased fuel.Furthermore, treated water from the WWTP can be utilized for slurryingwater for the fluid deoxidation unit. Moreover, the WWTP can also treatthe effluent from the deoxidation unit.

In particular, WWTP 201 receives storm drainage via one or more conduits203 and sewage via one or more conduits 204. Using known technology, aWWTP typically employs atmospheric air entering via a conduit 205 andvarious customary additives, such as flocculants and lime, via atransport system 206. This conventional treatment of sewage andwastewater results in the production of a digester gas, leaving the WWTPvia a conduit 207, which is utilized as a fuel source for the presentinvention. The treatment produces a viscous sewage sludge, i.e. a sludgeor slurry of biosolids leaving through a line 208. The concentration ofsolids will typically be in the range of between about 3% to 40% andaveraging about 20%. Because biosolids contain about 80% bound water,they are expensive to haul to acceptable disposal sites, to combust withthe water present, or to attempt to physically dewater them.

A deoxidation unit 202, employing the process of FIG. 1, is installed asclose as feasible to the source of the biosolids. By rupturing thecellular structure and splitting off carbon dioxide from the moleculesmaking up the biosolids, the slurry is readily mechanically dewatered tocontain about 35% to 65% solids. The now-separable (freed) water (about90% of that in the raw biosolids) is recycled to the WWTP through a line211, where it may be pretreated with membranes, ammonia removaltechnologies, anaerobic digestion technologies, or reverse osmosistechnologies. Upon drying, the char remaining has only about 15% to 17%of the weight of the raw biosolids, resulting in large cost savings fortransporting the char to a point of use or disposal.

Undried low-moisture char, exiting via suitable means 210, may beacceptable at a nearby landfill, to which it is transported by asuitable conveyor or carrier 212. It may similarly be transported to anearby incinerator, via a means 213, where its incineration will requiremuch less fuel than the corresponding raw biosolids would consume. Inaddition, either dried or undried char may be transported to a nearbycement kiln, via a means 214, where it requires significantly lesspurchased fuel than would be needed for an equivalent amount of rawbiosolids. The char may also be transported, via a means 215, to achemical plant where (aided by high reactivity) it is readily convertedto fuel or synthesized gas, to oxygenated compounds, to carbon fibers,to fertilizer production, and/or to landfill. The low-moisture char maybe transported, by a means 216, either as a pumpable slurry or as drypellets, to a thermal power station where its high reactivity permitsefficient combustion with low excess air and high carbon burnout.

Equally as significant as the flow of materials and energy is a flow ofmoney, in the form of a tipping fee, from the WWTP to the biosolidsprocessing unit, as indicated by a dashed line 217. The tipping fee isthe fee paid by the WWTP to the owner of the processing unit formanaging its biosolids.

Since the supply of the new fuel discussed above will initially besmall, it is optimal for local use. As such, one of the first fuel usersto accept it is likely to be cement kiln operators, since they can to alarge extent tolerate its high ash content. Other suitable areas of useare blast furnaces and foundries, since they are accustomed to firingcoal or coke and to disposing of ash with other impurities as slag. Asthe supply of biosolids char increases, it will become of interest togeneral coal users, including thermal power stations. Such applicationsare addressed in more detail in the remaining figures.

For example, FIG. 3 is a flow diagram illustrating an efficientbiosolids processing facility for converting biosolids into acombustible, preferably carbonized material that is combined with acement kiln. This aspect of the present invention highlights the drasticreduction of water that would otherwise accompany raw biosolids into thekiln, enabling a substantial increase in the amount of biosolidsconsumed, with a proportionate increase in tipping revenue received bythe processor and Btus charged to the kiln.

In particular, a fluid deoxidation unit 301, employing the processdescribed with respect to FIG. 1, is installed as close as feasible toone or more WWTPs, the source of the biosolids, as indicated by atransport means 303. By rupturing the biosolids cell walls anddischarging carbon dioxide that may be formed at the same time (line304), the resulting char can now be readily mechanically dewatered tocomprise about 35% to 65% solids. The now-separable water (about 90% ofthat in the raw biosolids) is recycled to the WWTP through a line 305 oris used as recycle water for process slurrying.

Char, either as a concentrated slurry, a wet solid, or a dried solid, istransported to a cement kiln 302 via a transport means 306. The basicingredients of Portland cement (limestone, clay and shale) are chargedvia conduits 307, 308 and 309, and are ground, mixed and charged to thekiln through a conduit 310. In a preheat section, these ingredients arecontacted counter-currently with hot flue gas, which raises thetemperature to drive off water of crystallization and calcine thelimestone. Near the bottom of the preheat section, waste combustibles,such as used tires and broken asphalt, are charged through a conduit311. If necessary to achieve the desired temperature, fuel such as coal,oil or gas is fired, together with combustion air, into the lower partof the preheat section. The preheated mix is then discharged into oneend of a horizontal, rotating kiln.

As the preheated ingredients travel to the opposite end of the rotatingkiln, they are further heated to the temperature necessary for them toreact and form cement clinker by firing, at a discharge end, primaryfuel delivered through a conduit 312 (which may include biosolids char),along with the corresponding combustion air supplied via a combustionair fan (not shown) and a conduit 313.

Flue gas, from which most of the sensible heat has been recovered,leaves the kiln via an exhaust fan and dust recovery equipment (notshown) through a line 314. Cement clinker exits the kiln via heatexchange with combustion air, through a conduit 315. The cooled clinkeris ground and blended with gypsum to form Portland cement.

Most of the ash constituents of biosolids char are tolerable in Portlandcement, with the exception of soluble cations, such as sodium andpotassium and the sulfates and chlorides, which go primarily to theeffluent from the liquid deoxidation unit and are returned via a conduit305 to the WWTP. The exception is phosphorus, which often is bound ininsoluble form by iron. It is possible that the phosphorus content couldlimit the amount of biosolids char a given cement kiln can accept.Should the content of phosphorus in the char produced by the unit 301 beso high as to limit the amount of biosolids char that can be accepted inthe cement clinker, a chelate solution (or other solublizing agent) maybe employed via a line 316 to extract some of this element. Thephosphorus-containing extract is then discharged through a line 317 andmust be disposed of in a manner that avoids returning it to the WWTP.

The inorganic fraction of biosolids can be as high as about 50% on a drybasis. This inherent ash found in biosolids can reduce the quantities oflimestone, clay and shale input in lines 307, 308 and 309, respectively.If unit 301 is located near the cement kiln 302, a portion of awastewater stream 305 can be utilized in the cement kiln 302 for coolingor other purposes, or in NOx reduction. Waste heat from the stream 314,or other waste heat streams, including radiation heat, can be utilizedby unit 301 as process heat for the system including heating the feedmaterial, process heat, or drying the reacted product. Evolved carbondioxide from the stream 302 can be conducted to the cement kiln 302 forheat recovery or odor reduction.

Equally as significant as the flow of materials and energy is a flow ofmoney, in the form of tipping fees, from the WWTP to the combination ofunits 301 and 302, as indicated by the dashed line 318. A portion of thefee goes to the owner of the unit 301, as indicated by a dashed line319, and the remainder goes to the owner of the cement kiln 302 asindicated by a dashed line 320.

FIG. 4 is a simplified flow diagram of an efficient biosolids processingfacility 401 employing deoxidation to convert biosolids into acombustible material in close proximity to and combined with a thermalpower station 402. The unit 401 is typified by FIG. 1, chargingbiosolids from a WWTP. However, since the supply of biosolids availableto a station of economic size is unlikely to be sufficient for its fuelneeds, it also represents a family of liquid deoxidation processescharging a spectrum of renewable biomass and/or hydrophilic low-rankfossil fuel. With any or all of these potential fuels, liquiddeoxidation makes them less hydrophilic and more uniform and thermallyefficient for combustion in the power station 402. The station 402represents a spectrum of conventional and unconventional combustionsystems culminating, via steam turbine or gas turbine combined cycles,in the production of electricity for the local market and/or thenational grid.

Biosolids are charged to the unit 401 via a line 403. Alternatively oradditionally, biomass waste, as paper mill sludge or from agriculture orforestry, is delivered by a transport means 404, and (optionally)hydrophilic low-rank fossil fuel is delivered through a transport means405. Water as required to form a pumpable charge slurry is added througha line 406. After being processed according to FIG. 1, the now excesswater is returned to a WWTP, or treated for discharge by known means,via a line 407. Uniform (dewatered) high energy density char slurry, ordried and pelletized char, is delivered through a transport means 408 tothe station 402.

The char or char slurry transported by the transport means 408 iscombusted by one of the known methods to yield thermal energy for thegeneration of steam, which is expanded through conventional steamturbines driving electric generators, or it may be partially oxidized(with either air or commercial oxygen) to yield a fuel gas subsequentlyburned in a gas turbine combustor driving an electricity generator, thehot exhaust gas from which generates steam for an integrated steamturbine-driven generator. The partial combustion of char may beaccomplished according to known processes separating the ash as a fluidslag, or in accordance with U.S. Pat. No. 5,485,728, the entiredisclosure of which is incorporated by reference herein, which teachesseparation of the ash particles in an aqueous slurry.

Since the amount of char available may have insufficient fuel energy togenerate the amount of electricity for which there is a market,supplemental fossil fuel can be supplied via a transport means 410. Airfor the combustion or partial combustion of the biomass and/or fossilfuel char is supplied through a line 411. After subjecting it toappropriate known pollution control measures, flue gas (or gases) fromthe combustion at the station 402 is (are) discharged through a stack412.

Treated boiler feed water makeup is supplied through a line 413, andblowdown required to maintain boiler water within specifications isdischarged via a line 414 to the unit 401, where it may comprise some ofthe water needed to form a sufficiently fluid feed slurry to thedeoxidation operation. Ash, the non-combustible residue from burning thechar and auxiliary fuels, or ash slurry, is withdrawn for disposal via aconduit 415.

One of the known methods of controlling the emission of nitrogen oxidesfrom atmospheric pressure boilers is overfiring with a reactive fuelabove the main flame zone. Because of its volatiles content and highreactivity, biosolids char is a suitable fuel for this purpose, and aportion of that from the conveying means 408 can be diverted by way of atransport means 416 for nitrogen oxide reduction. The product of thecombination, electricity, is delivered from the site via electric cables417.

For simplification, the biosolids treatment unit 401 is shown as thoughit had the capacity and raw material supply to furnish the power station402 with sufficient char fuel. In a practical installation, a treatmentunit 401 may be located adjacent to the power station 402, and one ormore such units 401 may be installed at other location(s) close to theraw material sources. This gives the operator the flexibility to employtailored deoxidation temperatures, optimized for the particularfeedstock. In such an event, dry char can then be shipped to the powerstation 402 by road or rail or, if economics dictate, it can be suppliedas an aqueous slurry via a pipeline. The flow of money, in the form of atipping fee, from the WWTP to the deoxidation unit, is indicated by adashed line 418

FIG. 5 is a simplified flow diagram of a combination comprising athermal dryer unit 501 and a cement kiln 502. The thermal dryer unit 501is installed as close as feasible to one or more cement kilns 502,employing principally the same configuration as shown in and describedin conjunction with FIG. 3, but without deoxidizing the biosolids.Biosolids are supplied via a transport means 503. By applying heat tothe raw biosolids cells, water contained in the cells is evaporated andleaves via a line 505 for scrubbing and condensing or, alternatively, isconducted via a line 517 back to the kiln to be utilized in the kiln asmake-up water or for NO_(x) reduction.

The resulting dried biosolids are conducted to the kiln via a line 506where the Btu value as well as the value of the ash are utilized. Theprimary ingredients, such as are shown in FIG. 3, are added to the kilnat the lines 507, 508, 509 through a conduit 510. As in FIG. 3, in apreheat section, waste combustibles are added, such as used tires andbroken asphalt charged through a conduit 511. As in FIG. 3, combustionair and primary fuel arrive via conduits 513 and 512, respectively.Cement klinker exits the kiln via a conduit 515.

Although thermal drying has an inherent energy penalty from the latentheat in the evaporation of water, this penalty can be entirely orpartially overcome by integrating with the cement kiln and utilizingheat from the kiln via a conduit 518. More specifically, flue gas,normally traveling via a conduit 514 to an appropriate discharge, can bedirected via a conduit 516 to the thermal dryer, thereby reducing theneed for primary fuels at thermal dryer 501 for evaporating the waterliberated from the biosolids.

As discussed briefly above, because the potential supply of biosolidschar is smaller, by orders of magnitude, than the general fuels market,other substances, for example biomass, can be co-processed in a liquiddeoxidation unit, or processed in parallel equipment, and the resultingchars blended before being used as a fuel, for example in accordancewith the teachings of U.S. Pat. No. 5,485,728. Several locations, suchas Hawaii (biosolids, pineapple and sugar cane wastes) and Sacramento,Calif. (biosolids and rice hulls and stalks), offer sites for slurry co-or parallel deoxidation. Paper mill and paper recycling sludges,although they may require alkali addition to neutralize chlorine, areother promising sources of supplemental hydrophilic biomass. Thesemethods afford a means of consolidating diverse sources into a uniformliquid or solid char slurry fuel.

EXAMPLES

The following examples are only representative of the methods andsystems for use in practicing the present invention, and are not to beconstrued as limiting the scope of the invention in any way.

Example 1

Biosolids from two wastewater treatment plants, one in Atlanta, Ga., andone in Riverside, Calif., were subjected to the earlier describedtreatment in a continuous pilot plant, resulting in the following feedand product analyses, reported on a moisture and ash-free basis:

Atlanta Raw Riverside Raw Biosolids Biosolids Carbon 57.73 62.53Hydrogen 7.74 9.26 Nitrogen 7.90 7.52 Sulfur 3.02 1.17 Oxygen 23.8619.52 Total 100.00 100.00

Atlanta Char Riverside Char Product Product Carbon 70.19 69.98 Hydrogen8.85 7.68 Nitrogen 8.63 8.45 Sulfur 1.42 8.86 Oxygen 10.91 5.04 Total100.00 100.00

As would be expected, the splitting off of carbon dioxide has resultedin an increase in carbon content and a corresponding decrease in oxygencontent.

The off-gas composition of the two runs was as follows:

Atlanta Riverside Off Gas Off Gas Carbon dioxide 89.7% 92.8% Volatileorganics 10.0% 6.0% Sulfur compounds 0.3% 1.2% Total 100.0% 100.0%

Theoretical Example 1

A cement kiln in the southwestern U.S. has a production capacity of3,200 tons/day. To reach temperatures required to form cement “clinker”,it fires low-grade coal, supplemented to some extent by charging scraprubber tires. Sensible heat in the flue gas, after preheating mineralcharge and combustion air, may be taken advantage of to dry andincinerate 20 tons/day (dry basis) of biosolids from area wastewatertreatment plants. Although every ton of dry biosolids constituents isaccompanied by about four tons of water (giving the biosolids a negativeheating value), revenue from the tipping fee offsets the cost of extracoal that must be fired. However, the amount is limited by the thermalcapacity to evaporate the water and by the increased volume of flue gas,increasing pressure drop and fan horsepower.

Using this invention, the kiln may use biosolids dewatered anddeoxidized in accordance with the present invention at one or more ofthe nearby WWTPs. As such, about 80% to 94% of the water formerlycharged with the raw biosolids bypasses the kiln, permitting it tocharge seven times as much deoxidized material without exceeding thermalcapacity and fan horsepower limits. The biosolids disposed of by thekiln can be increased by a factor of about 700%, with a correspondingincrease in tipping fees.

Other than in the operating examples, or unless otherwise expresslyspecified, all of the numerical ranges, amounts, values and percentagessuch as those for amounts of materials, times and temperatures ofreaction, ratios of amounts, and others in the following portion of thespecification may be read as if prefaced by the word “about” even thoughthe term “about” may not expressly appear with the value, amount orrange. Accordingly, unless indicated to the contrary, the numericalparameters set forth in the following specification and attached claimsare approximations that may vary depending upon the desired propertiessought to be obtained by the present invention.

Notwithstanding that the numerical ranges and parameters setting forththe broad scope of the invention are approximations, the numericalvalues set forth in the specific examples are reported as precisely aspossible. Any numerical value, however, inherently contains certainerrors necessarily resulting from the standard deviation found in theirrespective testing measurements. Furthermore, when numerical ranges ofvarying scope are set forth herein, it is contemplated that anycombination of these values inclusive of the recited values may be used.

The invention described and claimed herein is not to be limited in scopeby the specific embodiments herein disclosed, since these embodimentsare intended as illustrations of several aspects of the invention. Anyequivalent embodiments are intended to be within the scope of thisinvention. Indeed, various modifications of the invention in addition tothose shown and described herein will become apparent to those skilledin the art from the foregoing description. Such modifications are alsointended to fall within the scope of the appended claims. All patentsand patent applications cited in the foregoing text are expresslyincorporated herein by reference in their entirety.

What is claimed is:
 1. A process of converting biomass into a renewablefuel comprising the steps of: providing biomass comprising at leastabout 10% water; applying sufficient pressure to the biomass to maintainliquidity and form pressurized biomass; heating the pressurized biomassto a first temperature, wherein the first temperature is sufficient toform an aqueous biomass char slurry, carbon dioxide, and free water;depressurizing the biomass char slurry; separating the carbon dioxidefrom the biomass char slurry; removing at least a portion of the freewater from the biomass char slurry to provide a dewatered biomass charproduct containing a decreased oxygen content; and utilizing thedewatered biomass char product as a fertilizer.
 2. The process of claim1, wherein the biomass comprises sewage sludge.
 3. The process of claim1, wherein the first temperature is between about 200° C. and 345° C.(400° F. and 650° F.).
 4. The process of claim 1, further comprising thestep of adding an agent for dissolution of at least one polluting orslag-forming element present in the dewatered biomass char product. 5.The process of claim 4, wherein the agent comprises an alkali.
 6. Theprocess of claim 1, wherein a portion of the freed water is recycled tothe adding step.
 7. The process of claim 1, further comprising the stepof cooling the aqueous biomass char slurry to a second temperature lessthan the first temperature.
 8. The process of claim 1, wherein thesecond temperature is about 40° C. to 90° C. (100° F. to 200° F.). 9.The process of claim 1, further comprising the step of discharging thecarbon dioxide through a flame of at least one of an oxidizer and aprocess heater.
 10. The process of claim 1, further comprising the stepof slurrying the biomass by performing at least one of the steps ofgrinding and adding at least one of fresh water, recycled water, steam,process water from an adjacent wastewater treatment plant and acombination thereof to form a pumpable slurry.
 11. The process of claim1, further comprising the step of pretreating at least a portion of thefree water to form pretreated water and recycling the pretreated waterto an adjacent wastewater treatment plant.
 12. The process of claim 1,further comprising the step of using a digester gas from an adjacentwastewater treatment plant as fuel for the heating step.
 13. The processof claim 1, wherein the removing step comprises mechanically dewateringthe biomass char slurry to about 35% to 65% solids for use as thefertilizer.
 14. The process of claim 1, wherein the dewatered biomasschar product that is used as the fertilizer is undried.
 15. The processof claim 1, further comprising the step of subjecting the freed waterfrom the removing step to digestion.
 16. The process of claim 1, whereinthe biomass comprises about 3% to 40% solids.
 17. The process of claim16, wherein the biomass comprises about 20% solids.
 18. A process ofconverting biomass into a renewable fuel comprising the steps of:providing biomass comprising at least about 10% water; applyingsufficient pressure to the biomass to maintain liquidity and formpressurized biomass; heating the pressurized biomass to a firsttemperature, wherein the first temperature is sufficient to form anaqueous biomass char slurry, carbon dioxide, and free water;depressurizing the biomass char slurry; separating the carbon dioxidefrom the biomass char slurry; removing at least a portion of the freewater from the biomass char slurry to provide a dewatered biomass charproduct containing a decreased oxygen content; and directing thedewatered biomass char product to a landfill.
 19. The process of claim18, wherein the removing step comprises mechanically dewatering thebiomass char slurry to about 35% to 65% solids for use at the landfill.20. The process of claim 18, wherein the dewatered biomass char productthat is directed to the landfill is undried.